The present invention relates to a method and system for improving the quality of lithographic printing of integrated circuits on semiconductor wafers, and more particularly to insuring that a reticle used in the lithographic process is free of foreign particles which can adversely affect the printing results.
In lithographic printing, a reticle is a device which embodies a mask, or photomask, defining a design pattern to be transferred by lithographic printing onto a semiconductor wafer. Typically, a reticle is made of a material such as quartz, and may have dimensions, for example, on the order of several inches square by fractions of an inch thick. A film of, for example, chrome may be deposited on the reticle surface and processed to form the mask pattern.
In lithographic printing, the mask is interposed between a radiation source and a semiconductor wafer surface. The radiation passes through the photomask and impinges on an exposure field on the wafer which has been treated with a photosensitive resist material. The photomask pattern is transferred into the resist material, and the wafer is subsequently etched to effect a physical transfer of the mask pattern into the silicon.
Dimensions of the printed mask pattern are typically of a very small order. The size and spacing of some pattern features even approaches, or is less than, the wavelength of the radiation being used to print the pattern. Because of the small dimensions involved in printing the mask pattern, if foreign particulate matter existing in the ambient environment is present on the mask surface, it will interfere with the correct transfer of the mask pattern onto the resist surface.
Accordingly, the lithographic process typically comprises using a thin transparent membrane referred to as a pellicle to shield or seal off the photomask surface from the surrounding environment. The pellicle membrane is offset from the photomask surface, typically by a thin-walled rectangular pellicle frame which is adhered to the reticle. The pellicle membrane and frame create an enclosed air space above the photomask which ideally is free of foreign particles. Particles which subsequently settle on the outer surface of the pellicle membrane do not affect the printing result because they are offset from the focal plane of the lithographic optics.
In view of the importance of keeping the photomask particle-free, great efforts are undertaken to provide an environment for the reticles which is free of contaminants to the extent possible. Nevertheless, in spite of the precautions taken, even after the pellicle frame and pellicle membrane have been adhered to the reticle, foreign particles will be present on the photomask surface within the air space provided by the pellicle frame.
Once the pellicle frame is assembled and adhered to the reticle, an inspection process may be performed in which optical equipment is used to look through the pellicle membrane at the surface of the reticle to detect particles. If particles are detected, the pellicle assembly must be removed, which may damage the reticle and cause additional foreign matter to be introduced.
An existing method of controlling the amount of particulate matter present on the reticle involves coating the interior walls of the pellicle frame with an adhesive substance. Particles which happen to come into contact with the adhesive substance are captured on the sidewalls, reducing the amount of particulate contamination of the photomask surface to a certain extent. However, typically some particles will remain on the photomask even after the pellicle frame is adhered to the reticle, as described above.
In view of the foregoing discussion, a method and system are needed for removing particles from the photomask surface once the pellicle membrane, pellicle frame and reticle have been assembled.
A method and system according to the present invention provide for the removal of particulate matter from a photomask surface once the pellicle membrane, pellicle frame and reticle have been assembled. According to the invention, a pellicle frame is provided having an adhesive material on an interior wall. The pellicle frame and pellicle membrane are adhered to a reticle to form a finished reticle assembly.
The finished reticle assembly is placed on a device for dislodging any particles or foreign matter which may remain trapped within the pellicle air space over the photomask surface. The dislodging means causes the particulate matter to be dislodged from the photomask surface and move toward the adhesive interior walls of the pellicle frame, and become adhered thereto. The invention thereby permits particulate contamination to be removed from the photomask surface even after a finished reticle, pellicle frame and pellicle membrane have been assembled. Implementation of the invention following reticle assembly and routinely during the life of the reticle would extend reticle and pellicle life and reduce reticle/pellicle rework.